The invention relates to an artificial acetabulum made of an outer shell which houses an inner shell that cooperates with a spherical head of the joint.
Artificial acetabula are often made as double shells in which the outer shell permits attachment in osseous tissue, and the inner shell has a spherical surface to receive the spherical head of a prosthesis shaft. The outer and inner shells may be of different materials, to use, on the one hand, an outer shell which is tissue-tolerated as regards growing-in, and, on the other hand, to use for the inner shell materials, which have good properties as regards wear and anti-seizure behavior.
European patent publication EP 0 313 762 A1 is known an acetabulum in which the inner shell of plastics is fixed in the outer shell with a snap connection. The outer shell has a high rigidity which causes a high shape stability. The inner shell of plastics is elastically deformable, depending on the influence of forces acting through the spherical head, and is supported in this by the rigid outer shell. However, the rigid outer shell has the disadvantage that, in the case when the bone elastically yields under high compressional forces, tensile and shearing forces act on the side relieved of load between the outer shell and the bone.
European patent publication EP 0 445 068 A1 discloses a two-part acetabulum in which the outline of the outer shell may follow small changes of the supporting osseous tissue and thereby offers equalization of forces to the osseous tissue. Both shells are made of metal, the wall thickness of the outer shell being very thin compared to the wall thickness of the inner shell. The inner shell has the disadvantage that it must be made of metal to ensure a high shape stability of the inner surface on which the spherical head bears. The elastic outer shell has the disadvantage that, immediately after implantation, the in growth of bone is in some cases more difficult because during the growing-on relative movement might be possible. Furthermore, the elastic outer shell may follow the subsequently acting tensile and compressional forces between the outer shell and the bone only within the scope of its elastic pre-deformation during primary attachment.